Printing media are improved if they resist curl and remain flat. This is particularly important with media utilized with the formation with color images. Such media include those utilized for ink jet, thermal dye sublimation imaging, thermal wax dye transfer, and electrophotography. The formation of a flat base is particularly important for color photographic paper. In color papers there is a need for providing color papers with improved curl. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color paper as it is subjected to the developing and drying processes. Humidity changes during storage of color photographs lead to curling. There are particular problems with color papers when they are subjected to extended high humidity storage such as at greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl.
Curl values are the measurement amount of deflection in a parabolically deformed sample. An 8.5 cm diameter round sample of the composite is stored at the test humidity for 21 days. The amount of time required depends on the vapor barrier properties of the laminates applied to the moisture sensitive paper base, and it should be adjusted as necessary by determining the time to equilibrate the weight of the sample in the test humidity. The curl readings are expressed in ANSI curl units, specifically, 100 divided by the radius of curvature in inches.
The radius of curvature is determined by visually comparing the curled shape, sighting along the axis of curl, with standard curves in the background. The curl may be positive or negative, and for photographic products, the usual convention is that the positive direction is curling towards the photosensitive layer. It would be desirable if the humidity curl for photographic print paper could be reduced.
Color print papers are usually made of three structural layers under the emulsion; raw paper base and a top and bottom coating of extrusion cast polyethylene. Since the strength properties of polyethylene in this form are poor in comparison to the raw base, the curl properties or the stiffness of a print in bending are determined almost exclusively by the raw base. Because of the alignment of the paper fibers during manufacturing, the raw base is highly oriented with respect to strength properties in the plane of the sheet, for example the ratio of the elastic modulus in the strongest to weakest direction is often 2:1, with the strongest direction in the MD (machine direction). This is why the shape of a typical polyethylene sample with curl, caused by resisting emulsion shrinkage on one surface at low humidity, is cylindrical with the axis of curl along the MD, so the sample curls in its weakest direction, or directly in the CD (cross direction).